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BQ24780S: The selection of RC on ACDEC/ACDRV pin

Part Number: BQ24780S Hi team: I want to know the specific function of C1 C2，what’s the effect if I remove them? What are the requirements for their (C1 C2 R3 R4 )selection of value? Thanks! Alice

Hi Alice, C1 and C2 are used to control the turn-on speed of Q1 and Q2. If the turn-on speed is too fast, there is a large inrush current into the system capacitors and this could cause the adapter voltage to crash, which would be sensed as a disconnect at ACDET pin. If the turn-on speed is too slow, there is more heat generated and there is a danger of damaging the Q1 and Q2 FETs. Since different adapters have different capabilities and different FETs have different safe operating curves, the optimal turn-on speed is system dependent, but these capacitor values form a good balance. I would say that you might reasonably half or double them, but going outside that range has a good chance of causing one of the problems mentioned above. The R3 and R4 serve a different purpose. They limit current to protect the CMSRC and ACDRV pins during plugin transients. These values should not be modified. Regards, Steve